1. Field of the Invention
The present invention relates to a screening system for screening materials in order to prevent agglomeration, or to simply cause a break-up of the materials. More specifically, the present invention relates to a screen system for a centrifugal screening machine with a tube-shaped screen, and a procedure for operating a tube-shaped screen that causes increased amounts of screened material to pass through based on ultrasonic excitation of the screening system.
2. Description of the Related Art
In industry, there is a multiplicity of applications in which it is desirable to classify a material by size and/or keep particles from agglomerating, or breaking them up if they already have done so.
For this purpose, generally known screening machines are used which apply screening systems that differ particularly in the configuration and direction of the screen used. With the current vibration-type and tumbling-type screening machines, screens are used that have an essentially planar screen mesh kept in tension in a frame, the tension which in essence is directed perpendicular to the desired direction in which the material flows. The screened material that does not meet the classification condition pre-set by the openings of the screen mesh, stays back on the screen; only screened material that meets the classification condition can leave the screen.
While these classes of screening machines, based on the screening arrangement used by them, are able to be applied for batch-fed screened material, they are poorly suited for use when screened material is fed continuously. Particularly in these latter instances, centrifugal screening machines, also known under the designation of vortex flow screening machines, are used. One such screening machine, for example, is known from DE 30 19 113 C2.
With the type of screening machine designated in this patent application as a “centrifugal screening machine,” the screen system used has a tube-shaped screen, into the interior of which the screened material is fed. The tube-shaped screen can consist of a tube with screen openings that are directly placed in the wall of the tube; however, it can also be formed by a screen mesh that is placed in tension on a screen frame that defines at least the length and the cross section of a tube, so that the screen mesh forms a part of the surface of the tube-shaped screen and especially is configured not just in a plane. In addition, embodiment forms are also conceivable in which both a tube with screen openings, and an additional screen mesh that surrounds this tube, are provided.
A tube in accordance with this specification is an elongated hollow body with an opening running through it lengthwise and having a cylindrical cross section as a rule; accordingly, the adjective “tube-shaped” describes an object that has the form of a tube according to the definition used previously.
The tube-shaped screen acts achieves its screening action in that the material to be screened passes through the screen openings and/or the screen mesh that form at least a part of the tube wall. To ensure that considerable material passes through the screen openings and/or the screen mesh, there are two particularly expanded attachments. This is an option that is especially used with relatively small concentrations of the material to be screened, and consists in providing within the tube-shaped screen a fluid flow that transports the material, which is provided with one such flow rate, so that the material is transported through the screen openings and/or the screen mesh.
An alternative option, especially with high concentrations of material to be screened, consists in providing a so-called “impactor system” in the interior of the tube-shaped screen, i.e., a rotary gear manufactured as a rule out of metal, that is guided along the walls of the tube-shaped screen and pushes the screened material through the screen openings and/or the screen mesh and, if necessary, radial openings of the frame structure.
Generally with screen systems the problem is to prevent even temporary plugging of the screen openings and/or the screen mesh, as can happen for example by agglomeration of particles of screened material, and to ensure that screened material passes as efficiently as possible through the screen mesh. With screen systems that are used in centrifugal screening machines, this problem is exacerbated mostly because the screened material gets coated on the screen openings and/or the screen mesh, by an impactor system, for example.
For screen systems with planar screens in which the screen mesh forms a plane placed in tension in a screen frame, it is known to use ultrasonic excitation to reduce the tendency of the screen mesh to become clogged. One such screen system is known, for example, from DE 4418175. However, it is impossible to simply transfer this approach to screen systems with tube-shaped screens. The throughput does not significantly increase to what is desired.
Based on this prior art, the problem is to produce a screen system for a centrifugal screening machine with a tube-shaped screen, and a procedure for operating a tube-shaped screen that causes increased amounts of screened material to attainably pass through.
Accordingly, there is a need for an improved method and system for increasing throughput of screened material